The present invention relates to a method and to a control circuit for controlling the extinction ratio of a laser diode, and in particular, though not limited to a method and to a control circuit for controlling the extinction ratio of a laser diode of the type for use in optical transmission of digital data, for example, in telecommunications. The invention may also relate to a method and to a control circuit for controlling the average power output of a laser diode, and to a driver circuit for driving a laser diode.
Laser diodes are extensively used in optical transmission systems as light emitters for transmitting data in digital form over telecommunications networks. Such laser diodes have an optical power output/current input characteristic curve, which has an initial inactive linear portion which is almost horizontal with a very slight upward slope. The curve then has a knee portion which is generally called the threshold point at which stimulated light emission commences. The characteristic curve then continues as a relatively linear portion with a relatively steep slope. This, is referred to as the linear operating portion of the slope. A bias current is applied to the laser diode of value sufficient to maintain a predetermined average optical power output. A switched modulation current is applied to the laser diode to establish a predetermined extinction ratio.
However, the power/current characteristic curve tends to vary with temperature and also varies as the laser diode ages. Three typical power/current characteristic curves of a laser diode, namely, curves A, B and C are illustrated in FIG. 1. The curves A, B and C illustrate how the power/current characteristic curves vary with temperature. The curve A illustrates the power/current characteristic curve of the laser diode when operating at 0xc2x0 C. The curve B illustrates the characteristic curve of the laser diode operating at 25xc2x0 C., while the curve C illustrates the characteristic curve of the laser diode operating at 70xc2x0 C. An operating range of 0xc2x0 C. to 70xc2x0 C. is not an unusual operating temperature range for a laser diode. In each curve A, B and C the inactive portion is illustrated by the letter e. The threshold point of the curve is illustrated by he letter f, while the operating portion of each curve is illustrated by the letter g.
The input current in milliamps (mA) to the laser diode is plotted on the X axis of the graph of FIG. 1 while the optical power output in milliwatts (mW) of the laser diode is plotted on the Y axis. The bias current to the laser diode is indicated as Ib, while the modulation current is indicated as Im, The power output of the laser diode when the bias current Ib and the modulation current Im are applied to the laser diode is indicated as P1 while the power output of the laser diode when the only current applied to the laser diode is the bias current Ib is indicated as P0. The average power output of the laser diode is indicated as Pav which is equal to half the sum of P1 and P0, assuming an equal number of digital ones and zeros in the data stream. In order for the laser diode to operate efficiently, the bias current should be sufficient to operate the laser diode in the linear operating portion of the power/current characteristic curve just above the threshold point, in other words, the point h of the curve B, for example. Thereby the bias current operates the laser diode to produce a power output of P0. In this way, when the modulation current Im is applied to the laser diode on top of the bias current Ib the laser diode operates in the linear operating portion of the curve, namely, between the point h and the point k on the curve B. By operating the laser diode so that the power output varies between the points h and k on the characteristic curve B in response to the modulation current the laser diode operates with the optimum extinction ratio, which is the ratio of the power output P1 to the power output P0.
However, it will be clear from the curves A, B and C of FIG. 1 that should the operating temperature of the laser diode vary, unless the bias current Ib and the modulation current Im are varied to compensate for the change in operating temperature the laser diode will operate incorrectly. For example, if the bias current Ib and the modulation current Im were set to operate the laser diode at an operating temperature of 25xc2x0 C., an increase in the operating temperature would immediately cause the extinction ratio of the laser diode to drop, and also would result in a reduction in the average power output Pav of the laser diode.
Accordingly, in order for a laser diode to provide an adequate extinction ratio over its life and over a typical range of operating temperatures, control circuitry is required for altering the bias current Ib and the modulation Im to compensate for changes in operating temperature and as the laser diode ages. Typical control circuits which are known monitor the operating temperature of the laser diode, and alter the bias current and/or the modulation current in response to temperature change of the laser diode. A disadvantage of such circuits is that they tend to be inaccurate. They do not measure the extinction ratio directly. Measuring temperature gives only an indirect measure of the extinction ratio, and obviously, is not particularly accurate, since the extinction ratio as discussed may, in general, drift with age. Thus, any corrections made to correct the extinction ratio based on the operating temperature of the laser diode may be incorrect, thus leading to incorrect operation of the laser diode.
There is therefore a need for a method and a control circuit for controlling the extinction ratio of a laser diode.
According to the invention there is provided a method for controlling the extinction ratio of a laser diode, the method comprising the steps of
comparing a value of the power/current characteristic curve of the laser diode indicative of the slope of the operating portion of the power/current characteristic curve with a predetermined corresponding reference slope value, and
altering the modulation current to the laser diode in response to the slope value not comparing favourably with the reference slope value.
In one embodiment of the invention the slope value of the power/current characteristic curve is determined by altering the modulation current by a predetermined test amount, and determining the corresponding change in the average power output of the laser diode resulting from the alteration to the modulation current by the test amount.
Preferably, the predetermined test amount by which the modulation current is altered is of magnitude sufficiently small as not to affect normal operation of the laser diode.
Advantageously, the predetermined test amount by which the modulation current is altered is proportional to the modulation current when the modulation current is being altered. Ideally, the modulation current is increased by the predetermined test amount.
In one embodiment of the invention the predetermined test amount by which the modulation current is altered does not exceed 5% of the value of the modulation current when the modulation current is being altered, and preferably, the predetermined test amount by which the modulation current is altered does not exceed 1% of the value of the modulation current when the modulation current is being altered.
In another embodiment of the invention the modulation current to the laser diode is altered by a predetermined correcting amount in response to the slope value of the power/current characteristic curve not comparing favourably with the reference slope value. Advantageously, the correcting amount by which the modulation current is altered in response to the slope not comparing favourably with the reference slope value does not exceed 1% of the maximum value of the modulation current which may be applied to the laser diode.
In one embodiment of the invention the slope value of the power/current characteristic curve of the laser diode is compared with the reference slope value at periodic intervals.
In another embodiment of the invention the reference slope value is set and stored during calibration of the laser diode.
In a further embodiment of the invention the stored reference slope value is a reference value of the change in the average power output of the laser diode which should result from the alteration to the modulation current by the predetermined test amount if the laser diode were operating at the desired extinction ratio.
In one embodiment of the invention the method further comprises the step of controlling the average power output of the laser diode by comparing the average power output with a predetermined reference average power output value, and varying the bias current to the laser diode in response to the average power output not comparing favourably with the reference average power output value.
Preferably, the bias current to the laser diode is altered by a predetermined correcting amount in response to the average power output not comparing favourably with the reference average power output value.
Preferably, the comparison of the slope value with the reference slope value and the comparison of the average power output with the reference average power output value are alternately made.
Advantageously, the bias current to the laser diode is maintained constant during the period while the slope value of the power/current characteristic curve is being determined.
Additionally, the invention provides a control circuit for controlling the extinction ratio of a laser diode, the control circuit comprising:
a means for determining a value of the power/current characteristic curve of the laser diode indicative of the slope of the operating portion of the power/current characteristic curve,
a slope comparing means for comparing the value indicative of the slope of the power/current characteristic curve with a corresponding predetermined reference slope value, and
a first control means for controlling the modulation current to the laser diode, the first control means being responsive to the slope comparing means for altering the modulation current in response to the slope value not comparing favourably with the reference slope value.
In one embodiment of the invention the means for determining the value indicative of the slope of the power/current characteristic curve of the laser diode comprises:
a second control means for altering the modulation current to the laser diode by a predetermined test amount, and
a means for determining the corresponding change in the average power output of the laser diode resulting from the alteration of the modulation current by the test amount.
In another embodiment of the invention the slope comparing means compares the change in the average power output of the laser diode with the reference slope value.
Preferably, the second control means maintains the alteration to the modulation current by the test amount for a period sufficient to allow the resulting corresponding change in the average power output of the laser diode to be determined. Advantageously, the predetermined test amount by which the modulation current is altered by the second control means is such as not to affect the normal operation of the laser diode. Preferably, the predetermined test amount by which the modulation current is altered by the second control means is proportional to the modulation current when the modulation current is being altered. Advantageously, the second control means increases the modulation current by the predetermined test amount. Preferably, the predetermined test amount by which the modulation current is altered by the second control means does not exceed 5% of the modulation current when the modulation current is being altered. Ideally, the predetermined test amount by which the modulation current is altered by the second control means does not exceed 1% of the modulation current when the modulation current is being altered.
In one embodiment of the invention the first control means alters the modulation current by a predetermined correcting amount in response to the slope value not comparing favourably with the reference slope value. Preferably, the correcting amount by which the first control means alters the modulation current in response to the slope value not comparing favourably with the reference slope value does not exceed 1% of the maximum value of the modulation current which may be applied by the first control means.
In one embodiment of the invention the control circuit further comprises a power output comparing means for comparing the average power output of the laser diode with a predetermined reference average power output value, and a bias current control means for controlling the bias current to the laser diode, the bias current control means being responsive to the power output comparing means for altering the bias current to the laser diode in response to the average power output of the laser diode not comparing favourably with the reference average power output value.
Preferably, the slope comparing means and the power output comparing means are alternately operated for alternately comparing the slope value with the reference slope value and the average power output with the reference average power output value, respectively.
Advantageously, the bias current control means holds the bias current constant while the second control means is altering the modulation current by the predetermined test amount.
Preferably, a means is provided for setting the predetermined reference slope value.
Further the invention provides a laser diode driver circuit comprising:
a modulation current source for providing a current for modulation in response to a data stream to be transmitted by the laser diode,
a bias current source for providing a bias current to the laser diode,
a control circuit for controlling the extinction ratio of the laser diode, the control circuit comprising:
a means for determining a value of the power/current characteristic curve of the laser diode indicative of the slope of the operating portion of the power/current characteristic curve,
a slope comparing means for comparing the value indicative of the slope of the power/current characteristic curve with a corresponding predetermined reference slope value, and
a first control means for controlling the modulation current source, the first control means being responsive to the slope comparing means for altering the modulation current from the modulation current source in response to the slope value not comparing favourably with the reference slope value.
The advantages of the invention are many. The method and control circuit allow for accurate control of the extinction ratio of a laser diode. By monitoring and determining the slope of the operating portion of the power/current characteristic curve the operating characteristics of the laser diode are directly monitored, and thus, the extinction ratio can be accurately controlled. The extinction ratio may be corrected by making relatively small incremental or decremental changes to the modulation current as appropriate, and each incremental or decremental change may be of a predetermined correcting amount. After each incremental or decremental change, after checking the average power output of the laser diode and making an appropriate correction to the bias current if required, the slope value of the power/current characteristic curve is again determined and a further appropriate incremental or decremental change in the modulation current is made. In this way correction of the extinction ratio is made in a series of steps, each step correction being made by incrementing or decrementing the modulation current by a predetermined correcting amount. Alternatively, the control circuit may be provided with additional intelligence, which would allow computation of the full correcting amount by which the modulation current should be incremented or decremented in order to correct the extinction ratio, and the modulation current could then be altered by the appropriate correcting amount in one step.
A further advantage of the invention is that where a monitoring photo diode is used for monitoring the average power output and the change in average power output from the laser diode, the method and control circuit according to the invention operate independently from the capacitance of the monitoring photo diode, in other words, the capacitance of the monitoring photo diode has no affect on the method and control circuit. The method and control circuit also provide a relatively low band width non-peak detection method which facilitates ease of implementation. By virtue of the fact that the test amount by which the modulation current is altered is relatively small, and in particular, is a relatively small proportion of the modulation current no degradation is caused to the optical power output. A further advantage of the invention is achieved when the modulation current is altered by increasing the modulation current by the test amount, rather than by decreasing the modulation current by the test amount, in that the extinction ratio is slightly increased, rather than decreased, and this, thus, further avoids any degradation in the optical power output.
Additionally, the method and control circuit provide a relatively simple and accurate method and circuit for controlling the extinction ratio, and as well the average optical power output which has no affect on the data transmission. Additionally, the method and control circuit permit accurate correction of the extinction ratio without affecting data transmission, and thus, the extinction ratio can be corrected during normal operation of the laser diode. Furthermore, since in order to determine the slope of the power/current characteristic curve the modulation current only is altered by the test amount, the power output of the laser diode resulting from the bias current remains unaltered. Accordingly, laser chirp (change in wave length) is avoided during the period while the slope of the power/current characteristic curve is being determined and while correction if any is being made to the modulation current as the method for controlling the extinction ratio is being carried out.
The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawings.